eogrāfiski raksti folia geographica xii - Ģeogrāfijas un Zemes zinātņu ...

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NATURE RESEARCH
xA air and continental subpolar air cP would enable the existence of snow cover but would not
increase its thickness, although cP air may produce light snowfall. In the 1990s the coldest cA
air was identified only in the last week of December 1996. The temperature minimum over
snow covered territories dropped to -30º to -36ºC. The xA air affected Latvia each January, yet
it was not identified each December. The xA air brings the customary severe winter weather,
when the daily average temperatures may fall to -25° or -26°C. The first half of MW
(December) showed as high a frequency of continental air as the previous season (PW), but the
second half was very much dominated by mP air.
In mid-winter, a day or two may occur, when midlatitude (mSp, xSp) air arrives, and in
January 1998 even subtropical xS air reached Latvia. These very warm air masses together with
warmed subpolar air comprised together ca 30% (fronts excluded).
As a whole, MW produced precipitation < 60 mm/month. In the 11-year period, MW was
characterised by the lowest annual variation of the prevailing air mass frequency compared to
other seasons. The standard deviation of monthly near surface air and 850 hPa pseudopotential
temperatures were lower than in other winter months (Table 1, Figure 8). However, it should be
noted that the present study did not include any winter with an extended inflow of cA air
because 1987/1988 was the start of a period of relative mild winters. January 1987 is known to
be the coldest on 100-year record. An unusually cold outbreak of cA air from the north-east
brought near record low January temperatures (-37°C) and a record long period of very low
(-25° to -30°C) temperatures.
Late winter, LW (third decade of January to end of February)
In early February the incoming short-wave radiation starts gradually increasing and
doubles by the end of the month (98 MJ/m²). Regardless of the slight increase of solar radiation,
February was the coldest month in five of the 11 years. Late winter is dominated by subpolar
and arctic air masses. In the 11-year period the frequency of temperate/warm mP and xP air was
39% and that of winter air (xA, mA, cP) 32% (fronts excluded). With an exception of the
extremely warm February 1990 with only a couple of days of mA air, in late winter the
frequency of arctic air masses slightly increased. Frosts are common and, although rarely, the
cA air may affect Latvia. February 1994 was such a really cold month, with a minimum of –
29.4°C on the coldest day in Riga and even lower (-33°C) elsewhere in Latvia as a result of cA
air. Late winter shows an increased occurrence of transformed maritime air masses. The reason
for this is xA air, which is more frequent at this time than in any other month because a short
period of anticyclonic weather often occurs in early February. The anticyclones often form over
Northern Europe in maritime air of arctic origin, which gradually is transformed into xA air and
brings severe frosts in cold Februaries. The air mass t850 and θ850 are almost the same as in
January ( Figures 7, 8).
LW showed the lowest frequency of warmed subpolar air Ps, and also mid-latitude and
subtropical air.
In February of 1990, which was the warmest February on a 100-year record, when
precipitation was only in the form of rainfall, the unusual warmth “woke up” the dormant plants
and birch sap started rising in the last week of February. It was an extremely warm month due to
several spells of mid-latitude and even subtropical air. Average diurnal temperatures stayed
above zero almost the whole month except 16-19 February.
LW is a dry season. In the 11-year period, the mean monthly precipitation for February
was 35 to 55 mm over the territory of Latvia and 45.7 mm in Riga, comprising here 7% of
yearly precipitation. Unless anomalous warm weather occurs, a continuous snow cover is
common for February, which is maintained and renewed by winter air masses. On the whole,
February’s average temperatures showed more variation than any other month (10.5°C), and the
standard deviation of near surface air temperature and 850 hPa pseudopotential temperatures
θ850 was higher than in any other month (Table 1, Figure 8).